Amplitude Modulation (AM) & Single Sideband (SSB)
When you add two frequencies together, you get both of the original frequencies as well as the sum and difference of the two. In an AM transmitter, the carrier signal varies in strength relative to the audio input as opposed to Frequency Modulation which varies the frequency relative to the audio input.
The spoken voice is mostly between 300Hz and 3,000Hz and for the purpose of ham and CB, the audio portion of the signal will be between these two values.
In AM radio, you have the transmitter frequency (820Khz in the case of WBAP radio in Ft. Worth) You also have the sum and difference frequencies. If you put a steady tone of 500Hz audio onto WBAP's audio input, the result would be three signals. 819.5KHz, 820Khz, and 820.5KHz. The original audio (500Hz) is filtered out since it doesn't travel well anyway. (click the image below to enlarge)
Radio pioneers observed that the center frequency (820KHz) contains no information, yet uses over a third of the power that is sent out to the antenna. All of the information is contained in the side bands (sum and difference frequencies). The below example is from an AM broadcast station which is why the upper and lower sidebands are so much wider than in amateur radio signals. You can see at "A" the center frequency of the broadcast with a large amount of energy spent carrying no information. The upper and lower side bands "B" carry mirror images of the data payload.
Since the side bands are mirror images of each other, they are redundant to each other. The person transmitting Single Side Band (SSB) picks one. In our example, the Lower Side Band (LSB) has been selected. The person receiving needs to pick the same one or their signal becomes inverted (sounds a bit like monkey chatter). In the LSB example below, the carrier frequency "C" has been filtered out before transmitting because it carries no data. All of the information in the form of voice is intact in the LSB at "D". The upper side band (USB) has been filtered out before transmitting because the information there is an entirely redundant mirror image to that contained the LSB.
Let's say you are using lower side band, sending the 500Hz tone on 820KHz. (click the image below to enlarge)
The transmitter filters out the original 500Hz audio as well as both the 820Khz carrier and the 820.5KHz upper side band reducing the amount of power needed for a given signal strength by more than two thirds. This 819.5KHz "upper sideband" is amplified and sent to the antenna. (click the image below to enlarge)
The receiver is set to a local oscillator frequency of 820KHz. The received signal comes in at 819.5KHz and combined with the local oscilator signal. This again results in three frequencies. 500Hz (difference), 820KHz (local oscillator), 819.5Khz (received signal), and 1,639.5KHz (sum). (click the image below to enlarge)
The receiver filters out everything above 3,000Hz (the top of the voice range) and you are left with the 500Hz difference signal. (click the image below to enlarge)
In SSB mode, the transmitter is putting out almost no power when the microphone is keyed as long as there is no sound picked up by the microphone. In an AM transmitter, you still have more than 1/3 of the peak power being transmitted as long as the microphone is keyed regardless of whether there is any sound.
Radio pioneers observed that the center frequency (820KHz) contains no information, yet uses over a third of the power that is sent out to the antenna. All of the information is contained in the side bands (sum and difference frequencies). The below example is from an AM broadcast station which is why the upper and lower sidebands are so much wider than in amateur radio signals. You can see at "A" the center frequency of the broadcast with a large amount of energy spent carrying no information. The upper and lower side bands "B" carry mirror images of the data payload.
Since the side bands are mirror images of each other, they are redundant to each other. The person transmitting Single Side Band (SSB) picks one. In our example, the Lower Side Band (LSB) has been selected. The person receiving needs to pick the same one or their signal becomes inverted (sounds a bit like monkey chatter). In the LSB example below, the carrier frequency "C" has been filtered out before transmitting because it carries no data. All of the information in the form of voice is intact in the LSB at "D". The upper side band (USB) has been filtered out before transmitting because the information there is an entirely redundant mirror image to that contained the LSB.
Let's say you are using lower side band, sending the 500Hz tone on 820KHz. (click the image below to enlarge)
The transmitter filters out the original 500Hz audio as well as both the 820Khz carrier and the 820.5KHz upper side band reducing the amount of power needed for a given signal strength by more than two thirds. This 819.5KHz "upper sideband" is amplified and sent to the antenna. (click the image below to enlarge)
The receiver is set to a local oscillator frequency of 820KHz. The received signal comes in at 819.5KHz and combined with the local oscilator signal. This again results in three frequencies. 500Hz (difference), 820KHz (local oscillator), 819.5Khz (received signal), and 1,639.5KHz (sum). (click the image below to enlarge)
The receiver filters out everything above 3,000Hz (the top of the voice range) and you are left with the 500Hz difference signal. (click the image below to enlarge)
In SSB mode, the transmitter is putting out almost no power when the microphone is keyed as long as there is no sound picked up by the microphone. In an AM transmitter, you still have more than 1/3 of the peak power being transmitted as long as the microphone is keyed regardless of whether there is any sound.
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